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Amplitude Definition:
1.The maximum extent of a vibration or displacement of a sinusoidal (!) oscillation, measured from
the position of equilibrium. Amplitude is the maximum absolute value of a periodically varying
quantity.
2.The maximum difference of an alternating electrical current or potential from the average value.The term "amplitude" is used to refer to the magnitude of an oscillation, so the amplitude of the sinusoid "y = A × sin (ω×t)", is | A |, where | A | is the absolute value of A.The amplitude is a variable characterizing a sinusoidal oscillation. It gives the deflection of a
physical quantity from its neutral position (zero point) up to a positive or negative value.
The amplitude is expressed in a physical quantity − for example, as voltage, sound pressure, etc.
Amplitudes are expressed either as instantaneous values or mostly as peak values.
Amplitude is the fluctuation or displacement of a wave from its mean value. With sound waves, it is
the extent to which air particles are displaced, and this amplitude of sound or sound amplitude is
experienced as the loudness of sound.
From the "Encyclopedia Britannica": For a transverse wave, such as the wave on a plucked string,
amplitude is measured by the maximum displacement of any point on the string from its position
when the string is at rest. For a longitudinal wave, such as a sound wave, amplitude is measured by
the maximum displacement of a particle from its position of equilibrium. When the amplitude of a
wave steadily decreases because its energy is "being lost" (converted to heat), it is said to be
damped. Sound waves in air are longitudinal, pressure waves.

Drop a stone on a pond

Commonly it is spoken of "amplitude", as if there would be just a certain amplitude
as displacement or elongation from the zero-axis (baseline or equilibrium).
Amplitude can be a word that describes a wave. It means that maximum amount the
wave varies from the baseline or equilibrium. Displacement is usually used to
describe particles in motion, as in how far a particle has moved from a given point.
The wavelength in a longitudinal wave refers to the distance between two
consecutive compressions or between two consecutive rarefactions.
Definition: The amplitude is the maximum displacement from equilibrium. For a
longitudinal wave which is a pressure wave this would be the maximum increase (or
decrease) in pressure from the equilibrium pressure that is cause when a
compression (or rarefaction) passes a point.
The amplitude is the distance from the equilibrium position of the medium to a
compression or a rarefaction.
The peak value of sinusoidal AC signals is referred to as amplitude starting from the
zero line.
The amplitude usually refers to the scalar or vector field size.

http://uk.answers.yahoo.com/question/index?qid=20111025210805AAIaFhXQuestion of answers.yahoo:
Sound... What is amplitude?
I'm wondering what is it that creates the amplitude of a sound
wave?
I understand that as represented as a transverse wave, amplitude is the maximum value of
the wave function but how does this translate into longitudinal waves? It makes sense to me
that the shorter or further the distance between a high or low pressure pocket of air makes
the difference between a higher or lower frequency sound but seeing as the frequency
determines the pitch and the speed of sound is constant (depending on the medium) what is
it that provides a softer or louder sound? I've read that the intensity or energy of the sound
waves is what makes it louder or quieter, but if sound is travelling at the same speed, what
property of the wave as it travels through air is the term intensity or energy referring to?

Answer of answers.yahoo:
"Sound... What is amplitude?"
That is a really good question, because there is a problem with the definition of the
word amplidude.

Amplitude is the magnitude of change in the oscillating variable with each oscillation
within an oscillating system. For example, sound waves in air are oscillations in
atmospheric pressure and their amplitudes are proportional to the change in
pressure during one oscillation. If a variable undergoes regular oscillations, and a
graph of the system is drawn with the oscillating variable as the vertical axis and
time as the horizontal axis, the amplitude is visually represented by the vertical
distance between the extrema of the curve and the equilibrium value.
In older texts the phase is sometimes very confusingly called the amplitude.
Particle displacement is called particle amplitude. A transverse wave has an
amplitude. Particle velocity has an amplitude. Sound pressure or acoustic pressure
has an amplitude. Every (audio) frequency has an amplitude. A pendulum has an
amplitude.
Disputable information: "Sound intensity or acoustic intensity has an amplitude.
Sound power has an amplitude. Sound energy has an amplitude. Sound energy
density has an amplitude. Sound energy flux has
an amplitude." But these are sound
energy sizes.
The amplitude does not show directly the energy - The greater the amplitude the
greater is the energy. Energy = amplitude squared.
So what is amplitude? A "sound" has an amplitude. A loud sound has a bigger
amplitude than a soft sound. Which amplitude is really meant?
Cheers ebs

Answer of answers.yahoo:
Loudness depends on sound pressure, frequency, bandwidth, and duration.
Loudness is the quality of a sound that is primarily a psychological correlate of
physical strength (amplitude). It is a subjective measure, often confused with
objective measures of sound strength such as sound field sizes, like sound
pressure or sound pressure level SPL in decibels, and sound energy sizes like
sound intensity or sound power; see:
"Loudness - Wikipedia": http://en.wikipedia.org/wiki/LoudnessCheers ebs
PS: Don't forget that our eardrums are effectively moved by the "sound pressure";
see: "Sound pressure and Sound power − Effect and Cause":http://www.sengpielaudio.com/SoundPressureAndSoundPower.pdfWe measure the sound by an SPL meter (SPL = Sound Pressure Level).

Note: Time, frequency and phase belong close together.
The height of the amplitude has no influence on those parameters.The amplitude A has nothing to do with the frequency, the wavelength,
the time duration and the speed of sound.

The waveform parameters of a "117 V and 230 V RMS alternating current" sine wave form are summarized at the table below.

Average voltage

RMS voltage (VRMS)

Peak voltage (Vp) = (Û)

Peak-to-peak voltage (Vpp)

0 volts

117 volts = VRMS = ~V

165 volts = √2×VRMS = 0,5 × Vpp

330 volts = 2×√2×VRMS = 2 × Vp

0 volts

230 volts = VRMS = ~V

325 volts = √2×VRMS = 0,5 × Vpp

650 volts = 2×√2×VRMS = 2 × Vp

The value VRMS of an alternating voltage V (t) = V0 × f(t) is defined so that the
effective DC power corresponds VRMS2 / R = VRMS × IRMS to an ohmic resistance
of the middle resistive power of this AC voltage to the same resistance.

Enter one known voltage:

VRMS = VoltVp =VoltVpp = Volt

Do not reenter an answer!

The crest factor means the ratio of the peak voltage to the RMS voltage. If we need
to calculate an attenuator (attenuation calculation) we calculate a voltage divider.

Voltage conversions

VRMS = ~V

Vp

Vpp

Average voltage RMS VRMS =

−

0.7071 × Vp

0.3535 × Vpp

Peak voltage Vp =

1.414 × VRMS

−

0.5000 × Vpp

Peak-to-peak voltage Vpp =

2.828 × VRMS

2.000 × Vp

−

Unclear equations in books

The sound intensity I in W/m2 in a plane progressive wave
is given as: or also as But only one equation can be correct.Sometimes, these equations will show further information: or also as The tilde will indicate that it is the RMS value and the roof will show that it is the
amplitude value, ie, the peak value. For sinusoidal signals, the peak value means
the amplitude.
With these more accurate data, both equations are correct. You just need to know
exactly whether the peak value or the RMS value is applied.